Normally more than 3 days would be better but in that case, I think I would be tempted to blast down as far as possible on day 1 on the french autoroute (A75 over Millau is actually surprisingly fun for a motorway) and then try to take in a few of the best fast roads in Catalonia on the way to San Sebastian, something like this (see above map, you might need to click to make it load)

For an S1000R and a brisk ride, this route could be a great deal of fun.

Hi having done the Milau viaduct and the Pyrenees several times and going again this June I would recommend getting the ferry from Plymouth/ Portsmouth to Santander and then its a short trip to San Sebastian. Then I would take 3 days in the Pyrenees and head of to Milau then back up through France i only say this as France is a bit flat the higher up you get so can become a bit dull I think the more time In the Pyrenees the better
Alex